Gas compression systems are used in a wide variety of applications, including air compression for powering tools, gas compression for storage and transport of gas, and compression of refrigerant gases for refrigeration systems. In each system, motors are provided for driving the compression mechanism to compress the gas. The size and type of motor depends upon several factors such as the type and capacity of the compressor, and the operating environment of the system. Providing adequate motor cooling, without sacrificing energy efficiency of the compression system, continues to challenge designers of gas compression systems.
For example, motor cooling of compressor motors in refrigeration systems, especially large-capacity systems, remains challenging. In a typical refrigeration system, the compressor and the expansion device generally form the boundaries of two parts of the refrigeration circuit commonly referred to as the high-pressure side and the low-pressure side of the circuit. The low-pressure side generally includes biphasic piping connecting the expansion device and the evaporator, the evaporator, and a suction pipe that provides a path for refrigerant gas from the evaporator to the compressor inlet. The high-pressure side generally includes the discharge gas piping connecting the compressor and the condenser, the condenser, and the piping providing a path for liquid refrigerant between the exit of the condenser and the expansion device. In addition to the basic components described above, the refrigeration circuit can also include other components intended to improve the thermodynamic efficiency and performance of the system.
In the case of a multiple-stage compression system, and also with screw compressors, an “economizer” circuit may be included to improve the efficiency of the system and for capacity control. A typical economizer circuit for a multiple stage compression system includes means for drawing gas from a “medium-pressure” part of the compression cycle to reduce the amount of gas compressed in the next compression stage, thus increasing efficiency of the cycle. The medium-pressure gas is typically returned to suction or to an early compression stage. A cooling process for motors in a refrigeration system that includes an economizer is described in the U.S. Pat. No. 4,899,555.
Centrifugal compressors are often used for refrigeration systems, especially in systems of relatively large capacity. Centrifugal compressors often have pre-rotation vanes at their suction inlets that are used to vary the flow of refrigerant gases entering the compressor inlet. Centrifugal compressors are usually driven by electric motors that are often included in an outer hermetic housing that encases the motor and compressor. While this configuration reduces the risk of refrigerant leaks, it does not permit direct cooling of the motor using ambient air. The motor must therefore be cooled using a cooling medium, typically the refrigerant used in the main refrigerant cycle.
Many modes have been proposed and implemented to circulate refrigerant to cool compressor motors. For example, refrigerant can be sent in gas or liquid phase to the active parts of the motor and to the motor housing. In such cases, the refrigerant is necessarily supplied through orifices or passageways provided in the motor housing. After cooling the motor, refrigerant gas is typically sent to the compressor suction, either through paths internal to the compressor or through external pipes.
In some known motor cooling methods using liquid refrigerant, the refrigerant is sourced from the high-pressure liquid line between the condenser and the expansion device. The liquid is injected into the motor housing where it absorbs motor heat and rapidly evaporates or “flashes” into gaseous form, thus cooling the motor. The resulting refrigerant gas is then sent typically to the compressor suction through channels provided in the motor housing and/or in the motor itself. The benefit of liquid injection cooling is that there exists a great variety of potential injection points in a typical motor assembly. Other advantages of direct liquid cooling include the flow of liquid refrigerant over and around hard to reach areas such as the rotor and stator assemblies, thereby establishing direct contact heat exchange. Such direct contact heat exchange has been found to be a highly desirable method of cooling the motor in general, and particularly the rotor assembly and motor gap areas of the motor. Unfortunately, the high velocity liquid refrigerant sprays produced by known direct liquid refrigerant injection techniques represent a potentially dangerous source of erosion to exposed motor parts such as the exposed end coils of the stator winding. To avoid this problem, some manufacturers incorporate enclosed stator chambers to provide for motor cooling by indirect heat exchange, such as described in U.S. Pat. No. 3,789,249. In such assemblies, a sealed chamber or jacket is provided around the outer periphery of the stator, and low-velocity liquid refrigerant is circulated through the chamber to provide indirect heat exchange to the stator assembly. Such systems avoid the potential erosion problems of direct liquid refrigerant injection, but are not very effective in cooling other motor areas such as the air gap, rotor area, and the motor windings.
To avoid the risks of liquid refrigerant injection for motor cooling, it is also possible to use refrigerant gas. On small capacity refrigeration systems having small displacement compressors, the most common gas motor cooling method is to circulate all or most of the gaseous refrigerant to be handled by the compressor through the motor housing. Some gaseous refrigerant can also be taken at high pressure, or at medium pressure in the case of a multiple stage compressor. Refrigerant gas can be channeled into the motor and motor housing at various locations, and can be circulated using various modes. For example, U.S. Pat. No. 6,009,722 describes a way to circulate some cold gas from the evaporator transverse to the motor axis to cool the windings area. In contrast, U.S. Pat. No. 5,350,039 describes a way to circulate some high-pressure gas internally from the second stage impeller into the motor housing before it is released into the discharge pipe. The resulting gas circulation in the motor is axial in the provided air gap, stator notches, and passages around the stator.
A significant drawback of the above gas-phase motor cooling systems and methods is that usually, virtually the entire refrigerant gas flow is circulated through the motor and motor housing. There is much more refrigerant gas flowing through the motor than what is needed for cooling, and the gas flow through the motor generates substantial pressure drops that reduce the system efficiency. While such pressure drops and resulting inefficiencies may be acceptable for small capacity refrigerant systems, they are not acceptable or suitable for large capacity compressors. Accordingly, those systems are used in reciprocating compressors and small screw or scroll compressors, but not for large centrifugal compressors. For large capacity refrigeration systems, such as those used to cool office buildings, large transport vehicles and vessels, and the like, it is desirable to send only a limited amount of refrigerant to cool specific points of the motor and motor housing.
Another problem is the sourcing of the coldest available refrigerant gas through the motor housing to ensure adequate cooling. For example, it is possible to draw gas from the high-pressure side of the refrigeration circuit for cooling, and return it to the compressor suction. However, a relatively high gas flow is required because the relatively high gas temperature cannot provide efficient cooling of the motor. Also, the sourced gas must be re-compressed without providing any cooling effect in the cycle. Thus, the high-pressure side is a poor motor coolant source because of its severe effects on system efficiency.
Alternatively, it is possible to cool the motor using medium-pressure gas from an economizer cycle. Where an economizer is provided, medium-pressure gas can be sourced from a compression stage of the motor and returned to a lower compression stage or possibly to compressor suction. Sourcing and circulation of such medium-pressure gas is simple because of the substantial pressure difference available between medium and low pressures in the economizer and low-pressure side, respectively. While the problem of marginal motor cooling due to elevated gas temperature is still encountered, the required volume of gas flow is lower because of the lower relative gas temperature. Medium-pressure cooling systems, as described by U.S. Pat. No. 4,899,555, as well as by U.S. Pat. No. 6,450,781, have been implemented with limited success. In both of the medium-pressure gas cooling systems, the gas circulated through the motor housing is at medium pressure, resulting in higher gas friction than if the gas were taken at low pressure, further limiting the cooling effect on the motor.
In light of the foregoing, there is a continuing need for an efficient system and method for motor cooling in gas compression systems using the circulated fluid without adversely affecting system capacity or significantly reducing system efficiency.